Ace of Clubs

While it may not have the cache of the Tyrannosaurus, Brachiosaurus or Triceratops, Ankylosaurus is firmly settled in the pantheon of well-known dinosaurs.

The giant herbivore that lived in the late-Cretaceous Period — about 66 million years ago — cuts a memorable figure with its armored back reminiscent of a turtle shell. Its most famous feature, though, is a bony, clubbed tail that scientists believe Ankylosaurus used as a weapon.

That heavy medieval tail-club, however, is not a feature Ankylosaurus always had. The Ankylosaurus that most dinosaur fans know is one of the last of an ancient line of dinosaurs stretching back into the Jurassic Period. All of these ankylosaurs had the wide armored body that Ankylosaurus does, but the group lived for up to 70 million years before the first full tail-clubs showed up.

The “why” of the tail-club seems obvious: keep away the predators and any other dinosaurs trying to encroach on Ankylosaurus’s territory. Now in a new study, scientists from NC State University and the University of Alberta in Canada have combed through ankylosaur fossils from a period of 80 million years to figure out the “how.”

Victoria Arbour, a postdoctoral researcher at NC State and the North Carolina Museum of Natural Sciences and lead author of the study, says the tail-club has two essential parts, which she calls the “knob” and the “handle.” The knob is the bludgeoning club-head, and is composed of two huge bony growths in the ankylosaur’s skin called osteoderms.

The handle is actually a change in how the ankylosaur’s tail is built. Tails are generally built like the spinal cord, small bones stacked one on top of the other and lined with muscle that allow for great flexibility and range of motion. Picture how a cat or a monkey can curl and manipulate its tail.

The problem with that design is what happens when you put a large weight on one end and try to move it. All of that flexibility makes it harder to move, stop and control a heavy object. Imagine, for instance, a sledgehammer. With a handle, you can get good leverage on it, swing hard and stop it pretty much where you want to. If you were to replace that handle with rope, you could get it up to speed, but good luck stopping it or adjusting where it is going to hit.

An ankylosaur trying to swing it’s tail would have the same problem with the added issue that if the tail picked up too much momentum, it could pull the ankylosaur’s muscles or worse. Ankylosaurus got around this by having many of the vertebrae in its tail fused into a straight rigid structure, giving it more leverage and control over the knob.

Ankylosaurus, which lived in the late Cretaceous, had the knob and the handle, but its ancestors in the Jurassic Period had neither, so Arbour and her colleagues set to examining ankylosaurs to see which feature developed first or whether they developed together.

Arbour found that several species, including the Gobisaurus (left), which lived in the early Cretaceous 90 million years ago, had overlapping and fused vertebrae but no club: a handle but no knob. The first fully formed knob formed 15 million years later.

Arbour says that it is possible that in some species, the handle and knob and handle developed simultaneously, but in the species she examined, the trajectory always went handle first, then knob.

This makes sense as the ankylosaurs really would have needed the handle to support the knob. It does, however, present an interesting evolutionary story.

Evolution by natural selection is not purposeful, meaning that species do not design their own evolutionary traits. In this case, the ankylosaurs could not have gotten together for an ankylosaur meeting in the Jurassic Period and decided:

“Hey we could do some serious damage with a club tail, but first we need to stiffen our tails to support the club. Let’s spend about 60 million years making our tails more rigid, and in another 10 million we can grow a sweet club. All right. See everybody next week.”

Something about having a less flexible tail gave those ankylosaurs an advantage, and then tens of millions of years later, the club gave new ankylosaurs an advantage.

Today, animals use their tails to bat away insects or for balance, so a rigid tail might have provided some advantage that proved more valuable than performing those two tasks at full capacity.

Maybe that advantage was having a huge stick to hit other dinosaurs with but it could have been anything. It eventually became a weapon — which became more effective with the evolution of the club — but it did not necessarily start that way.

Regardless of why the ankylosaurs needed a rigid tale, they developed one, and that rigid tale allowed for the creation of one of the most iconic and fascinating traits of any dinosaur: Ankylosaurus’s clubbed tail.

The research appears in the Journal ofAnatomy.

— Daniel Lane

Daniel Lane covers science, engineering, medicine and the environment in North Carolina.